Systems and methods for reducing the likelihood of inducing collateral neural activity during neural stimulation threshold test procedures

ABSTRACT

Test procedures for determining a neural stimulation threshold of a patient. In one embodiment, the procedure includes applying a test stimulation signal to the patient and monitoring the patient for a response to the test stimulation signal. The procedure can further include determining a first neural stimulation threshold and calculating a second neural stimulation threshold. The first neural stimulation threshold corresponds to the lowest intensity test stimulation signal that evokes a patient response. The second neural stimulation threshold corresponds to a treatment stimulation signal directed toward affecting a neural activity within the patient.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No. 10/350,947, filed Jan. 24, 2003, now U.S. Pat. No. 7,024,247 which is a Continuation-in-Part of U.S. application Ser. No. 09/978,134, filed Oct. 15, 2001, now U.S. Pat. No. 7,305,268 which is a Continuation-in-Part of U.S. application Ser. No. 09/802,808, filed Mar. 8, 2001, now U.S. Pat. No. 7,010,351 which claims priority to U.S. Provisional Application No. 60/217,981, filed Jul. 13, 2000, all of which are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure is related to systems and methods for reducing the likelihood of inducing collateral neural activity while determining threshold parameters for electrically stimulating a region in the cortex or other area of the brain.

BACKGROUND

A wide variety of mental and physical processes are controlled or influenced by neural activity in particular regions of the brain. The neural-functions in some areas of the brain (i.e., the sensory or motor cortices) are organized according to physical or cognitive functions. There are also several other areas of the brain that appear to have distinct functions in most individuals. In the majority of people, for example, the areas of the occipital lobes relate to vision, the regions of the left interior frontal lobes relate to language, and the regions of the cerebral cortex appear to be consistently involved with conscious awareness, memory, and intellect.

Many problems or abnormalities with body functions can be caused by damage, disease and/or disorders in the brain. Effectively treating such abnormalities may be very difficult. For example, a stroke is a very common condition that damages the brain. Strokes are generally caused by emboli (e.g., obstruction of a vessel), hemorrhages (e.g., rupture of a vessel), or thrombi (e.g., clotting) in the vascular system of a specific region of the brain, which in turn generally cause a loss or impairment of a neural function (e.g., neural functions related to facial muscles, limbs, speech, etc.). Stroke patients are typically treated using various forms of physical therapy to rehabilitate the loss of function of a limb or another affected body part. Stroke patients may also be treated using physical therapy plus an adjunctive therapy such as amphetamine treatment. For most patients, however, such treatments are minimally effective and little can be done to improve the function of an affected body part beyond the recovery that occurs naturally without intervention.

Neural activity is governed by electrical impulses or “action potentials” generated in and propagated by neurons. While in a quiescent state, a neuron is negatively polarized and exhibits a resting membrane potential that is typically between −70 and −60 mV. Through chemical connections known as synapses, any given neuron receives from other neurons excitatory and inhibitory input signals or stimuli. A neuron integrates the excitatory and inhibitory input signals it receives, and generates or fires a series of action potentials when the integration exceeds a threshold potential. A neural firing threshold may be, for example, approximately −55 mV. Action potentials propagate to the neuron's synapses, where they are conveyed to other neurons to which the neuron is synaptically connected.

The neural activity in the brain can be accordingly influenced by electrical energy that is supplied from a man-made source such as a waveform generator. Various neural functions can thus be promoted or disrupted by applying an electrical current to the cortex or other region of the brain. As a result, researchers have attempted to treat damage, disease and disorders in the brain using electrical or magnetic stimulation signals to control or affect brain functions. One treatment approach, transcranial electrical stimulation (TES), involves placing an electrode on the exterior of the scalp and delivering an electrical current to the brain through the scalp and skull. Another treatment approach, transcranial magnetic stimulation (TMS), involves producing a high-powered magnetic field adjacent to the exterior of the scalp over an area of the cortex. Yet another treatment approach involves direct electrical stimulation of neural tissue using implanted electrodes.

A neural stimulation signal may comprise a series or train of electrical or magnetic pulses that can affect neurons within a target neural population, and may be defined or described in accordance with stimulation signal parameters including pulse amplitude, pulse frequency, duty cycle, stimulation signal duration, and/or other parameters. Electrical or magnetic stimulation signals applied to a population of neurons can depolarize neurons within the population toward their threshold potentials. Depending upon stimulation signal parameters, this depolarization can cause neurons to generate or fire action potentials. Neural stimulation that elicits or induces action potentials in a functionally significant proportion of the neural population to which the stimulation is applied is referred to as supra-threshold stimulation; neural stimulation that fails to elicit action potentials in a functionally significant proportion of the neural population is defined as sub-threshold stimulation. In general, supra-threshold stimulation of a neural population triggers or activates one or more functions associated with the neural population, but sub-threshold stimulation by itself fails to trigger or activate such functions. Supra-threshold neural stimulation can induce various types of measurable or monitorable responses in a patient. For example, supra-threshold stimulation applied to a patient's motor cortex can induce muscle fiber contractions.

Although electrical or magnetic stimulation of neural tissue may be directed toward producing an intended type of therapeutic, rehabilitative, or restorative neural activity, such stimulation may result in collateral neural activity. In particular, neural stimulation delivered beyond a certain intensity, level, or amplitude can give rise to seizure activity and/or other types of collateral activity, which may be undesirable and/or inconvenient in a neural stimulation situation.

Seizure activity may originate at a seizure focus, which is a collection of neurons (e.g., on the order of 1000 neurons) exhibiting a characteristic type of synchronous firing activity. In particular, each neuron within a seizure focus exhibits a firing response known as a paroxysmal depolarizing shift (PDS). The PDS is a large magnitude, long duration depolarization that triggers a neuron to fire a train or burst of action potentials. Properly functioning feedback and/or feed-forward inhibitory signaling mechanisms cause an ensuing afterhyperpolarization, through which the neuron's membrane potential returns to a hyperpolarized state below its firing threshold. Following the afterhyperpolarization, the neuron may undergo another PDS.

The afterhyperpolarization limits the duration of the PDS, thereby helping to ensure that synchronous neural firing activity remains localized to the seizure focus. Inhibitory feedback signaling provided by neurons surrounding a seizure focus, commonly referred to as surround inhibition, is particularly important in constraining seizure activity to the seizure focus. In the event that inhibitory signaling mechanisms fail and/or are unable to overcome or counter PDS activity, neurons within the seizure focus recruit other neurons to which they are synaptically coupled into their synchronous firing pattern. As a result, synchronous firing activity spreads beyond the seizure focus to other areas of the brain. This can lead to a cascade effect in which seizure activity becomes increasingly widespread and accompanying clinical manifestations become increasingly significant.

In view of the foregoing, it may be very important in any given neural stimulation situation to determine an appropriate stimulation signal amplitude, level, or intensity. However, an appropriate stimulation signal level may vary on a per-patient basis and possibly over time for any particular patient. Notwithstanding, determination of a neural stimulation threshold corresponding to a minimum or near-minimum stimulation signal level that induces or generates a measurable or monitorable patient response can provide a reference point for establishing a stimulation signal intensity appropriate for a neural stimulation session.

Various types of neural stimulation thresholds exist. For example, an electromyography or electromyographic (EMG) threshold may be defined as a lowest or near-lowest level of neural stimulation that generates an EMG signal of a particular magnitude. An EMG signal provides a measurement of electrical discharges associated with the innervation of muscle fibers by one or more motor neurons, and the onset of muscle fiber contraction in response to such electrical discharges. As another example, a sensation threshold may be defined as a lowest or near-lowest level of neural stimulation at which a patient notices, perceives, or experiences a physical sensation such as a tingling or vibration in a muscle group or limb. As yet another example, a movement threshold may be defined as a lowest or near-lowest level of neural stimulation that induces a noticeable movement in a patient's limb.

Unfortunately, neural stimulation threshold testing can itself induce collateral neural activity. During a typical neural stimulation threshold test procedure, a very low amplitude test stimulation signal is initially applied to a patient. The amplitude of the test stimulation signal is then increased incrementally, while other test stimulation signal parameters (e.g., frequency, pulse characteristics, duty cycle, etc . . .) remain unchanged or unmodified. As its amplitude is increased, the test stimulation signal is delivered to the patient in an uninterrupted or continuous manner. A lowest or near-lowest test stimulation signal amplitude that evokes a given type of patient response is correspondingly defined as the neural stimulation threshold. The patient is subsequently treated using a stimulation signal having parameters identical to those of the test stimulation signal, with the possible exception of stimulation signal amplitude, which may be a predetermined value based on the neural stimulation threshold.

Stimulation signal characteristics and manners in which stimulation signals are applied to a target neural population can significantly affect the likelihood of inducing collateral neural activity. Conventional neural stimulation threshold test procedures fail to adequately address this consideration, and thus may be susceptible to inducing seizure activity and/or other types of collateral neural activity. Hence, there is a need for systems and methods that reduce the likelihood of inducing such activity during neural stimulation threshold testing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system for neural stimulation threshold testing according to an embodiment of the invention.

FIG. 2 is a graph illustrating several parameters that may describe or characterize a stimulation signal.

FIG. 3 is a flowchart illustrating various methods for neural stimulation threshold testing according to an embodiment of the invention.

FIG. 4 is a flowchart illustrating various methods for neural stimulation threshold testing according to another embodiment of the invention.

DETAILED DESCRIPTION

The following disclosure describes systems and methods for reducing the likelihood of inducing collateral neural activity during the application of test stimulation signals to a patient for the purpose of determining a threshold stimulation level that induces or evokes a particular type of patient response, behavior, activity, sensation, perception, and/or reaction. In the context of the present invention, collateral neural activity may comprise seizure activity and/or essentially any other type of neural activity that may be undesirable, unwanted, unintended, and/or counterproductive relative to an intended or desired neural activity associated with neural stimulation threshold testing.

Various methods in accordance with the present invention are directed toward neural stimulation threshold test procedures that temporally manage the application of test stimulation signals to a patient. Neural stimulation lasting beyond several seconds (e.g., approximately 5 seconds) can lead to prolonged neural firing responses known as afterdischarges. Afterdischarges in a target neural population increase or tend to increase neural activity in the population, which may excite other neurons synaptically coupled to the target population. Afterdischarges can therefore increase the likelihood of inducing collateral neural activity. Neural stimulation threshold test procedures in accordance with the present invention may therefore apply limited duration test stimulation signals to patients.

Neural stimulation applied to a target neural population can affect the future excitability or firing likelihood of neurons in the target population. In particular, recently stimulated neurons may exhibit enhanced excitability in the presence of subsequent stimulation. A neural population that receives sub-threshold stimulation during a first time period may exhibit an increased firing susceptibility in the presence of same stimulation applied during a second time period when the second time period and the first time period are separated by only a brief interval (e.g., seconds or several seconds). Moreover, neural activity, such as afterdischarges, may persist for many milliseconds or seconds after a stimulation signal is interrupted or terminated. Hence, neural stimulation threshold test procedures in accordance with the present invention may additionally or alternatively provide for a quiescent time interval between successive threshold determination attempts during which test stimulation signals are not applied or delivered to the target neural population.

Various methods in accordance with the present invention may additionally or alternatively be directed toward neural stimulation threshold test procedures in which test stimulation signals and a treatment stimulation signal used to treat a patient may parametrically differ beyond their amplitudes. For example, depending upon stimulation signal parameters, neurons within a target population may respond to a stimulation signal in a synchronous manner. In particular, the ability of neurons to synchronously respond to or follow a stimulation signal is a function of the stimulation signal's frequency. Neurons can readily follow a stimulation signal up to a frequency of approximately 100 Hertz, but beyond this frequency their ability to follow the stimulation signal degrades. Supra-threshold stimulation delivered to a target neural population at a frequency that—neurons can readily follow may drive synchronous firing activity within the population. In view of the foregoing, a neural stimulation threshold test procedure in accordance with the present invention may measure a first neural stimulation threshold using one or more test stimulation signals having a first frequency; and subsequently calculate a second neural stimulation threshold corresponding to a treatment stimulation signal that may have a second frequency different from the first frequency. The calculation of the second neural stimulation threshold may be performed in accordance with one or more transformation equations, as described in detail below.

FIG. 1 is a schematic illustration of a system 100 for neural stimulation threshold testing according to an embodiment of the invention. In one embodiment, the system 100 comprises a stimulus unit 120 configured to deliver stimulation signals to a patient 190 through a patient interface 109. The system 100 may further include a sensing unit 180 coupled to the patient 190 and the stimulus unit 120.

The stimulus unit 120 is capable of generating and outputting stimulation signals, which comprise electrical and/or magnetic signals or stimuli. The stimulus unit 120 may perform, direct, and/or facilitate neural stimulation threshold test procedures in a manner that reduces the likelihood of inducing collateral neural activity in accordance with the present invention. Neural stimulation threshold test procedures may involve the application of one or more test stimulation signals to a patient 190 in manners described in detail below. The stimulus unit 120 may additionally perform, direct, and/or facilitate neural stimulation treatment procedures to treat a particular neurological condition and/or affect or influence a given type of neural activity. Neural stimulation treatment procedures involve the application of a treatment stimulation signal to a patient. The treatment stimulation signal, for example, is typically at a level or amplitude that corresponds to a result obtained during neural stimulation threshold test procedures (e.g., 20% to 80% of a neural stimulation threshold).

The stimulus unit 120 may comprise a controller 130, a pulse system 140, and a set of controls/indicators 150. The controller 130 may include a processor, a memory, and a programmable computer medium. The controller 130 may be implemented as a computer or microcontroller, where the programmable medium comprises software loaded into the memory, and/or hardware that performs, directs, and/or facilitates neural stimulation threshold test procedures in accordance with the methods of the present invention. The controls/indicators 150 can include a display device, an input/output device, and/or other types of devices for exchanging commands and/or output with a computer.

The pulse system 140 can generate energy pulses, and send such pulses to the patient interface 109. In one embodiment, the pulse system 140 forms a portion of a Transcranial Magnetic Stimulation (TMS) device through which externally applied magnetic stimulation signals create electrical currents in the patient's brain. In such an embodiment, the patient interface 109 may comprise an electromagnetic coil arrangement in a manner understood by those skilled in the art. In another embodiment, the pulse system 140 forms a portion of an electrical stimulation device; in this case the patient interface 109 may comprise an electrode array configured to deliver electrical stimulation signals to the patient 190 as described in detail hereafter.

The patient interface 109 shown in FIG. 1 comprises an electrode array 110 including a support member 112 and a plurality of electrodes 114 carried by the support member 112. The electrode array 110 is generally implanted into the patient 190 and configured for cortical stimulation, deep brain stimulation, and/or other types of neural stimulation. The electrode array 110 may comprise a cortical neural-stimulation device, such as a device described in U.S. application Ser. No. 09/802,808, incorporated herein by reference. The electrodes 114 may be coupled to the stimulus unit 120 by a link 116, which may be wire-based or wireless.

The electrode array 110 and the pulse system 140 can be integrated into a single implantable stimulation apparatus, as described in U.S. application Ser. No. 09/082,808. An integrated pulse system 140 and electrode array 110 may be configured for implantation into a patient's skull such that the electrodes 114 can contact the patient's dura matter or pia matter in a given cortical region. Such a device can have an internal power source that can be implanted into the patient 190, and/or an external power source coupled to the pulse system 140 via electromagnetic coupling or a direct connection. In alternate embodiments, the pulse system 140 is an external unit that is not implanted into the patient 190. An external pulse system 140 can provide stimuli to the electrodes 114 using RF energy, electromagnetism, or wire terminals exposed on the patient's scalp.

The sensing unit 180 may comprise a system or apparatus for measuring or monitoring one or more types of patient reactions evoked or induced in response to test stimulation signals applied during neural stimulation threshold test procedures. The sensing unit 180 can be coupled to the stimulus unit 120 by at least one link 186, which may be wire-based and/or wireless. The stimulus unit 120 may issue a signal over the link 186′ to synchronize the application of test stimulation signals to the patient 190 with sensing unit measuring, monitoring, and/or recording operations. Depending upon embodiment details, the sensing unit 180 may also use the link 186 to communicate status information and/or measurement results to the stimulus unit 120.

In one embodiment, the sensing unit 180 comprises an EMG device coupled to a plurality of electrodes or leads 182. The EMG device may detect or monitor motor evoked potentials (MEPs) associated with muscle fiber innervation in a manner understood by those skilled in the art. An EMG threshold may be defined, for example, as a lowest or near-lowest level of neural stimulation that induces an MEP that departs from baseline electrical activity by an amplitude greater than 50 microvolts peak-to-peak under 1000× amplification and 20-1000 Hertz bandpass conditions. The electrodes 182 may comprise surface, percutaneous, and/or implanted probes, which may be positioned or configured to measure electrical activity associated with one or more muscles or muscle groups. In one embodiment, the electrodes 182 include a ground lead and bipolar surface leads configured to monitor MEPs in an interosseus muscle, a wrist extensor, a wrist flexor, and/or other muscles.

Various embodiments of the present invention may alternatively or additionally detect or determine other types neural stimulation thresholds, such as a sensation threshold, a movement threshold, an Electroencephalogram (EEG) threshold, a Magnetoencephalogram (MEG) threshold, and/or an imaging threshold. The structure and/or function of the sensing unit 180 may correspond to the type of neural stimulation threshold under consideration. For example, to facilitate detection of a movement threshold, the sensing unit 180 may comprise a set of motion detectors, accelerometers, and/or strain gauges configured to detect or monitor one or more types of patient movements. To detect or determine an EEG threshold, the sensing unit 180 may comprise an EEG system configured to monitor changes in a patient's EEG during neural stimulation threshold test procedures. Such an EEG system may include and/or utilize electrodes positioned upon the patient's scalp, and/or intracranially upon a brain surface or in a subcortical region.

To detect or determine an MEG threshold, the sensing unit 180 may comprise an MEG system configured to monitor variations in the patient's MEG in response to test stimulation signals. To measure or determine an imaging threshold, the sensing unit 180 may comprise a neural imaging system configured to monitor and image a patient's neural activity during neural stimulation threshold test procedures. Suitable neural imaging systems include Magnetic Resonance Imaging (MRI) systems, functional MRI (fMRI) systems, or Positron Emission Tomography (PET) systems.

Different types of neural stimulation thresholds may vary with respect to measurement or observation subjectivity, and/or sensitivity to stimulation signal intensity. In general, determination of an EMG threshold may be a less subjective process than determination of a movement threshold, which may be a less subjective process than determination of a sensation threshold. Also, an EMG threshold may be triggered or induced at a lower stimulation signal intensity than a sensation or a movement threshold. In general, a sensation threshold may be triggered or induced at a lower stimulation signal intensity than a movement threshold, although this need not always be the case. Determination of movement and/or sensation thresholds may involve or rely upon human perception and verbal and/or visual feedback, and may not require the use of a sensing unit 180.

As previously indicated, the stimulus unit 120 is configured to deliver stimulation signals to a patient 190; the stimulation signals may comprise test stimulation signals and/or treatment stimulation signals. FIG. 2 is a graph illustrating several parameters that may define, describe, or characterize stimulation signals. A stimulus start time t_(o) defines an initial point at which a stimulation signal is applied to the patient interface 110. In one embodiment, the stimulation signal may be a biphasic waveform comprising a series of biphasic pulses, and which may be defined, characterized, or described by parameters including a pulse width t₁ for a first pulse phase; a pulse width t₂ for a second pulse phase; and a pulse width t₃ for a single biphasic pulse. The parameters can also include a stimulus repetition rate 1/t₄ corresponding to a pulse repetition frequency; a stimulus pulse duty cycle equal to t₃ divided by t₄; a stimulus burst time t₅ that defines a number of pulses in a pulse train; and/or a pulse train repetition rate 1/t₆ that defines a stimulus burst frequency. Other parameters include a peak current intensity 11 for the first pulse phase and a peak current intensity I₂ for the second pulse phase. Those skilled in the art will understand that pulse intensity or amplitude may decay during one or both pulse phases, and a pulse may be a charge-balanced waveform. Those skilled in the art will further understand that in an alternate embodiment, pulses can be monophasic or polyphasic.

FIG. 3 is a flowchart illustrating various methods for neural stimulation threshold testing according to an embodiment of the invention. In one embodiment, a method 300 may include a parameter determination procedure 302 involving determination, selection, or specification of test stimulation signal parameters that may reduce a likelihood of inducing collateral neural activity. Depending upon embodiment details, one or more types of test stimulation signal parameter sets, selections, and/or settings may be preprogrammed into the stimulus unit 120. A stimulus unit operator (e.g., a medical professional) may select a particular set of test stimulation signal parameters using the stimulus unit's controls/indicators 150. Exemplary test stimulation signal parameter selections that may reduce the likelihood of inducing collateral neural activity are described in detail below with reference to FIG. 4.

The method 300 may additionally include a signal management procedure 304 that involves managing the temporal application of test stimulation signals to a patient 190 in a manner that reduces the likelihood of inducing collateral neural activity. A stimulus unit operator may use the stimulus unit's controls/indicators 150 to initiate the signal management procedure 304. Particular manners of temporally managing test stimulation signal application in accordance with the present invention are described in detail below with reference to FIG. 4. The method 300 may additionally include an observation or measurement procedure 306 that involves observing and/or measuring one or more neural stimulation thresholds based upon one or more patient reactions and/or behaviors induced in response to the test stimulation signals. The observation procedure 306 may be performed using the sensing unit 180 and/or human perception and feedback.

The method 300 may further include a calculation procedure 308 involving calculation, mapping, and/or determination of a neural stimulation threshold corresponding to a treatment stimulation signal that is to be applied to the patient 190 for treating a neurological condition and/or affecting neural activity. The treatment stimulation may be parametrically distinct from one or more test stimulation signals with respect to one or more parameters. Operations performed and/or directed by the calculation procedure 308 may be based upon one or more measured and/or observed neural stimulation thresholds. The calculation procedure 308 may involve the use of one or more conversion or mapping functions and/or data tables stored in a memory of the stimulation unit. Such functions and/or data tables may be based upon known relationships and/or one or more empirical measurement histories capable of correlating test stimulation signal parameters with treatment stimulation signal parameters for a range of neural stimulation thresholds as measured or observed in association with the observation procedure 306.

FIG. 4 is a flowchart illustrating various methods for neural stimulation threshold testing according to another embodiment of the invention. In one embodiment, a method 400 includes a parameter determination procedure 402 that involves determining, specifying, and/or selecting test stimulation signal parameters that may reduce a likelihood of inducing collateral neural activity. In one embodiment, the pulse repetition frequency within a test stimulation signal may be higher than that within a treatment stimulation signal in order to reduce or minimize the likelihood that a significant number of neurons within a target neural population can synchronously respond to or follow the test stimulation signal. In general, a test stimulation signal needs to activate few or relatively few neurons within a target population to invoke or elicit a measurable and/or observable patient response. Thus, the pulse repetition frequency of a test stimulation signal may be higher than a neural frequency that produces a significant degradation in behavior or function (e.g., above approximately 100 Hertz), without adversely affecting the likelihood that a sufficient number of neurons can fire and evoke a patient response. In accordance with the present invention, exemplary test stimulation signal pulse repetition frequencies may be approximately 250 Hertz to approximately 400 Hertz, and in particular at the endpoints of this range.

An average amount of electrical current, charge, or energy delivered to a target neural population increases with increasing pulse repetition frequency, given constant or essentially constant test stimulation signal duration. As a result, a test stimulation signal having a higher pulse repetition frequency may be expected to elicit or evoke a measurable and/or observable patient response at a lower current level than an equivalent duration test stimulation signal having a lower pulse repetition frequency.

The method 400 may further include a signal application procedure 404 that involves application or initiation of application of (a) a first limited duration test stimulation signal having a low or very low intensity or amplitude to a target neural population within the patient 190; or (b) a next limited duration test stimulation signal having an incrementally or slightly higher intensity or amplitude than a previous test stimulation signal to the target neural population. The use of limited duration test stimulation signals may reduce or minimize the likelihood of generating prolonged neural firing responses such as afterdischarges that can increase neural excitation outside of the target population to which the test stimulation signals are applied. In general, a limited duration test stimulation signal in accordance with the present invention may be shorter than approximately 5 seconds.

The duration of a test stimulation signal applied in accordance with the present invention may depend upon a type of neural stimulation threshold currently under consideration. Thus, a limited duration test stimulation signal applied when determining one type of threshold may be shorter or longer than a limited duration test stimulation signal applied when determining another type of threshold. In general, a shorter duration test stimulation signal may deliver a lower average amount of electrical current, charge, or energy to the target neural population than a longer duration test stimulation signal given equivalent or generally equivalent pulse repetition frequencies. Hence, a shorter duration test stimulation signal may require a higher electrical current intensity, level, or amplitude to induce or evoke a given type of patient response.

With respect to determination of a movement threshold, a shorter duration test stimulation signal may evoke or induce a sharper, better defined, and hence more easily observable patient movement than a longer duration test stimulation signal. With respect to sensation threshold determination, the duration of a test stimulation signal may need to be sufficient to enable a patient 190 to accurately perceive and/or confirm perception of an induced sensation. Thus, in one embodiment, the signal application procedure 404 may apply test stimulation signals lasting at least approximately 1 second or less when the method 400 involves determination of a movement threshold; and/or apply test stimulation signals lasting approximately 3 seconds when the method 400 involves determination of a sensation threshold.

An EMG threshold may be determined in accordance with a variety of EMG measurement and/or EMG signal analysis techniques. The signal application procedure 404 may apply test stimulation signals having a duration corresponding to an EMG measurement and/or EMG signal analysis technique currently under consideration. In accordance with the signal application procedure 404, the stimulus unit 120 may issue a synchronization signal to the sensing unit 180 coincident or essentially coincident with the output of a test stimulation signal to initiate EMG signal monitoring or recording operations in a manner understood by those skilled in the art.

In one embodiment, the signal application procedure 404 may apply test stimulation signals to the patient 190 lasting approximately several milliseconds or on the order of tens of milliseconds when the sensing unit 180 is configured for EMG threshold measurement. An exemplary test stimulation signal duration corresponding to EMG threshold determination may be approximately 16 milliseconds. In such an embodiment, the signal application procedure 404 may repeat the application of a given test stimulation signal to the patient 190 multiple times to increase an EMG signal to noise ratio (i.e., to average out noise); repeated applications of the test stimulation signal may be separated by a minimum quiescent time interval as further described below. The determination of an EMG threshold using test stimulation signals having a duration on the order of milliseconds or tens of milliseconds may performed in a manner analogous or generally analogous to conventional nerve conduction studies. In another embodiment, the signal application procedure 404 may apply test stimulation signals to the patient 190 lasting approximately 1 second to 3 seconds.

In addition to the signal application procedure 404, the method 400 may include a monitoring procedure 406 that involves monitoring or observing the patient 190 and determining whether a patient behavior or reaction has been evoked or induced in response to the most recently applied test stimulation signal. The monitoring procedure 406 may involve the sensing unit 180 and/or human perception and feedback. In the event that the method 400 involves determination of an EMG threshold, the monitoring procedure 406 may record and/or measure an EMG response during one or more portions of a test stimulation and/or throughout its entirety. The monitoring procedure 406 may additionally or alternatively perform EMG signal analysis operations to enhance MEP detectability. Such EMG signal analysis operations may include determination of changes in MEP firing or activation rates, determination of changes in MEP activation complex durations or temporal widths, root-mean-square (RMS) amplitude analysis, power spectrum analysis, correlation analysis, and/or one or more other statistical analyses.

The method 400 may also include an adjustment procedure 408 involving modification or adjustment of test stimulation signal intensity or amplitude if no patient response was evoked in association with the signal application procedure 404. In the absence of an evoked response, test stimulation signal intensity may be increased in accordance with a particular increment, for example, by 0.5 or 1.0 milliamps, or by a given percentage. The method 400 may additionally include a waiting procedure 410 involving waiting or pausing for a minimum quiescent time interval following application of a most recent test stimulation signal to the patient 190. After the minimum quiescent time interval has elapsed, the method 400 may return to the signal application procedure 404.

The use of a minimum quiescent time interval after the application of a given limited duration test stimulation signal may reduce or minimize the likelihood that neurons within the target neural population will have an increased firing susceptibility or re-excitation likelihood during application of a subsequent test stimulation signal. A minimum quiescent time interval may range from several seconds to several minutes. In an exemplary embodiment, the minimum quiescent time interval is approximately 1 minute. In an alternate embodiment, the minimum quiescent time interval may be shortened or increased depending upon a cumulative number of test stimulation signals that had been applied to the patient 190.

The method 400 may additionally include a threshold measurement procedure 420 involving measurement or determination of a neural stimulation threshold corresponding to the most recently applied test stimulation signal that evoked or induced a patient reaction or response. In one embodiment, the neural stimulation threshold corresponding to a given test stimulation signal is the electrical current level, intensity, or amplitude at which the test stimulation signal evoked or induced a particular type of patient response.

The method 400 may further include a threshold calculation procedure 422 involving calculation of a neural stimulation threshold corresponding to a treatment stimulation signal to be applied to the patient. In calculating a neural stimulation threshold corresponding to a treatment stimulation signal, the threshold calculation procedure 422 may use one or more transformation formulas and/or conversion relationships, which may be programmably stored within the stimulus unit 120. Such transformation formulas and/or conversion relationships may be based upon known parameter relationships and/or empirical data measured across a variety of test stimulation signal parameter configurations, and they may be generated using curve fitting and/or numerical modeling procedures. In one embodiment, a transformation formula appropriate for test stimulation signals and treatment stimulation signals approximately 3 seconds or longer in duration may be of the following general form: I ₂(f ₂)=I ₁(f)*(1+k*(f ₁ **q))/((1+k*(f ₂ **q))  [1]

In the above equation, I₂ may be a peak, average, or RMS current level that defines or establishes a calculated neural stimulation threshold corresponding to the treatment stimulation signal; f₂ is a pulse repetition frequency associated with the treatment stimulation signal; I₁ is a measured neural stimulation threshold corresponding to a test stimulation signal; f₁ is a pulse repetition frequency corresponding to this test stimulation signal; and k and q are constants. The values of k and q may depend upon the nature of the test stimulation signals and/or the treatment stimulation signal. In an exemplary embodiment, for anodic monopolar test stimulation signals, k and q may respectively equal −0.9637 and 0.0249. For bipolar test stimulation signals, k and q may respectively equal −1.0047 and 0.0032.

The above conversion formula may scale linearly, quasi-linearly, or nonlinearly for test stimulation signal durations shorter than 3 seconds; the manner of scaling may depend upon how closely a test stimulation signal's duration approaches 3 seconds. In general, those skilled in the art will understand that additional and/or other types of transformation formulas may be defined, derived, and/or determined in accordance with particular test and/or treatment stimulation signal parameter characteristics under consideration.

A calculated neural stimulation threshold may be larger or smaller than the measured or observed neural stimulation threshold, depending upon test stimulation signal parameters and treatment stimulation signal parameters. The treatment signal may be parametrically distinct from the test stimulation signals; in particular, the treatment stimulation signal may have a different (e.g., lower) pulse repetition frequency and/or a longer (possibly continuous) duration than the test stimulation signals. The treatment stimulation signal may be delivered to the target neural population at an intensity, level, or amplitude that is a particular function or fraction of the calculated neural stimulation threshold.

Various methods for neural stimulation threshold testing in accordance with the present invention may employ additional, fewer, and/or other procedures than those described above. For example, test stimulation signals may differ from a treatment stimulation signal with respect to additional or other parameters than those described above. As another example, a procedure may include a step of verifying a calculated neural stimulation threshold by applying a treatment stimulation signal to the patient 190 at the calculated threshold, and determining whether a patient reaction occurs that is equivalent or generally equivalent to that which occurred during test stimulation signal application. A procedure may also record and store event information, data, and/or measurements obtained during test stimulation signal application. The disclosure herein provides for these and other variations, and is limited only by the following claims. 

What is claimed is:
 1. A neuro-stimulation system for application of cortical stimulation to a patient, comprising: an electrode array having an implantable support member configured to be implanted below the skin and above the dura of the patient's brain, wherein the electrode array has a plurality of therapy electrodes carried by the support member, a pulse system operatively coupled to the therapy electrodes, the pulse system delivering a stimulus to the therapy electrodes; a controller operatively coupled to the pulse system, the controller including a computer operable medium containing instructions that generate command signals that define the stimulus delivered by the pulse system and a desired stimulus to be delivered to the therapy electrodes based upon feedback input to the controller; a sensing device configured to be attached to a sensing location of the patient, the sensing device generating response signals defining the feedback input into the controller; and wherein the computer operable medium contains instructions causing the controller to perform the following method: applying an electrical stimulus having a plurality of stimulus parameters to the therapy electrodes; sensing a response to the applied electrical stimulus using the sensing device; determining whether the response is within a desired range or an improvement over a previous sensed response from a different electrical stimulus; selecting an alternate electrical stimulus; repeating the applying, sensing, determining and selecting procedures using the alternate electrical stimulus; and choosing an electrical stimulus corresponding to a sensed response that is within a desired range and/or is an improvement compared to other sensed responses.
 2. The system of claim 1 wherein the sensing device comprises a sense electrode configured to be attached to the patient at a sense location to sense a response to the stimulus applied to the therapy electrodes.
 3. The system of claim 1 wherein the sensing device comprises a sense electrode configured to be attached to the patient at a sense location and an EMG unit coupled to the sense electrode.
 4. The system of claim 1 wherein the sensing device comprises a functional MRI device that detects locations of neural-activity in the brain.
 5. The system of claim 1 wherein the sensing device comprises a sense electrode configured to be attached to the patient at a sense location and an electroencephalogram unit coupled to the sense electrode.
 6. The system of claim 5 wherein the sense electrode is positioned upon the patients scalp or intracranially upon the patient's brain. 